The stress energy tensor of a perfect fluid is composed of two terms of which only one term contains the metric tensor gab. (product of metric tensor and pressure). For curved spacetime, one replaces the flat spacetime metric tensor by the metric tensor of curved space. What I find bizar however is that the metric tensor enters into the expression of the stress-energy tensor in such an asymmetric way (one part affected, the other not). The same applies for the stress energy tensor of the electromagnetic field. Is there some deep reason why one part is affected while the other part is not ? Should one not expect that the geometry of spacetime affects all components in a similar way ?